This is the Roller-Walker, a four-legged robot which can either walk conventionally, or skate on retractable wheels. At the end of each leg there is a wheel, which can turn 90 degrees to act as a foot. On rough surfaces the robot walks, but if it comes across a smoother surface then it can skate in a weirdly graceful swimming motion. See a video of it in action after the jump.
The Roller-Walker has been constructed by the Hirose-Fukushima Robotics Lab in Japan, and uses considerably less energy when skating than conventional walking robots. [Robots.net]